Archaeologists have discovered a prehistoric hill fort dating back to 400BC on the North York Moors.

English Heritage archaeologists used global positioning technology to locate the 40-acre site at Sutton Bank near Thirsk.

Suspicions that a hill fort existed in the area date back to the mid-19th Century, when an Ordnance Survey team mapped a short stretch of "tell-tale" Iron Age earthworks.

Alastair Oswald, English Heritage archaeological field investigator, said the team was suprised to discover the site, thought to be one of the largest in the country.

'Tantalising clues'

"Hill forts are uncommon in Yorkshire, so we were obviously shocked to discover such a huge complex.

"Over the years there have been tantalising clues that such a monument existed, but no one had done a comprehensive survey to settle the question."

The fort would have been hard to breach

In the past, archaeologists have attempted to look for the fort but confused these earthworks with nearby boundary ditches of medieval origin.

English Heritage was prompted to investigate the mystery further following environmental work undertaken by the North York Moors National Park.

During the summer a team re-surveyed the area around Roulston Scar and the famous White Horse of Kilburn.

They discovered a series of ancient ditches, banks, humps and other features extending much further than anyone had previously thought.

Some stretches of the defences survive to a height of nearly three metres.

Mr Oswald added: "It's possible the fort was constructed by the Brigantes or Parisi tribes, perhaps more as a statement of power than a defensive bastion.

"We didn't find much evidence of settlement within its boundaries, but it may have been a temporary refuge in times of trouble, or a giant corral for livestock.

Important monument

"What is for certain is that such a large fort would have taken a vast amount of timber and labour to build which poses many more intriguing questions."

The survey revealed that the fort was enclosed by a two-metre-deep trench and a four-metre-high "box rampart", fronted by a timber palisade and topped by a defended walkway.

Only two entrances were detected, adding to the site's impregnability.

Individual features visible on the surface were carefully plotted onto a map using co-ordinates generated by the Global Positioning System, which uses signals from orbiting satellites to get a precise geographic fix.

The National Park Authority is considering how best to present this important prehistoric monument to the thousands of walkers who pass through on the Cleveland Way.